The present invention relates to a process for the production of argon from a cryogenic air separation process. In particular, the present invention relates to a process in which argon can be recovered substantially free of nitrogen.
A common method of recovering argon from air is to use a double column distillation system consisting of a higher pressure column and lower pressure column which are thermally linked with a reboiler/condenser and a side-arm rectifier column attached to the lower pressure column. The oxygen product is withdrawn from the bottom of the lower pressure column and at least one nitrogen-enriched stream is withdrawn from the top of the lower pressure column. A portion of the vapor rising through the lower pressure column is withdrawn from an intermediate location and passed to the side-arm column. This vapor portion, which generally contains between 5% and 15% argon by molar content and traces of nitrogen with the balance being oxygen, is rectified in the side-arm column to produce as an overhead, an argon-enriched stream. Typically, this argon-enriched stream, commonly, referred to as crude argon, is withdrawn from the top of the side-arm column with an oxygen content ranging from parts per millions levels to about 3% by molar content. The rectification is achieved by providing liquid reflux to the side-arm column via a condenser located at the top of the side-arm column.
Since nitrogen is more volatile than argon, most of the nitrogen contained in the side-arm column feed exits the side-arm column in the crude argon. Nitrogen is generally considered an impurity of an argon product, therefore, it is essential to limit the nitrogen content in the side-arm column feed. While the lower pressure column may be designed to virtually eliminate nitrogen from the side-arm column feed, in actual operation, some nitrogen is generally present. For example, plant upsets and flow ramping often cause the composition profile in the lower pressure column to shift from the design point to one in which nitrogen is present in the vapor portion fed to the side-arm column. Additionally, the reboiler/condenser located at the bottom of the lower pressure column could have small leaks which allow nitrogen from the higher pressure side to enter the column in a region which, by design, should be essentially nitrogen-free.
Since complete elimination of nitrogen from the side-arm column feed is difficult to achieve, it is widely accepted that nitrogen will be present in the crude argon withdrawn from the top of the side-arm column. As a consequence, the crude argon withdrawn from the side-arm column is typically subjected to an additional separation step by feeding it to a distillation column containing both rectifying and stripping sections, a reboiler located at its bottom and a condenser located at its top. Numerous patents exist in the art which describes such a column. See, for example, U.S. Pat. No. 5,590,544.
Many have reported that the nitrogen content of the crude argon withdrawn from he side-arm column may be reduced by withdrawing the crude argon from an intermediate location of the side-arm column.
Japanese Patent No. 07133982 discloses that the nitrogen content of the crude argon can be reduced by withdrawing said crude argon from an intermediate location of the side-arm column and removing nitrogen in a second, vapor purge stream taken from the top of the side-arm column. In Japanese Patent No. 07146066, an additional separation column is added to further treat the withdrawn crude argon, presumably, in recognition that not all the nitrogen may be reliably eliminated from the argon simply by withdrawing the stream from an intermediate location of the side-arm column.
U.S. Pat. No. 5,557,951 and DE-19636306-A2 disclose the practice of withdrawing the crude argon from the side-arm column at an intermediate location. In both these disclosures, there are no additional separation steps applied to the crude argon for the purpose of further removing nitrogen. Therefore, successful application of these disclosures requires that the nitrogen content of the side-arm column feed be kept below a threshold value.
As the off-design operation of the lower pressure column may cause the nitrogen content of the side-arm column feed to increase above the design level, the off-design operation of the side-arm column may also cause the nitrogen content of the crude argon to increase even though a vapor purge stream is employed. For example, it is critical that the nitrogen be allowed to exit the top of the side-arm column in the vapor purge stream. In practice, this stream can contain significant quantities of argon as well. Hence it is desirable to minimize the flow of the vapor purge stream to reduce argon losses. Unfortunately, restricting the flow of this vapor purge stream causes nitrogen to accumulate in the side-arm column, potentially causing nitrogen to appear in the crude argon.
The present invention allows for the production of substantially nitrogen-free argon in a cost effective and operationally sound manner.